Prior to processing further the rolled strip, the surface of the strip must be free of scale. For hot rolled strips, for instance, scale is formed as a so-called secondary scale when passing through the hot rolled strip finishing line on the cooling roller table, as well as also in the reeling installation where the strip is wound into coils or bundles. Depending on the quality of the material, the final thickness of the hot rolled strip and rolling speed, the scale attains different layer thicknesses, as well as a different chemical composition.
Acid pickling units today are predominantly used for removing the scale from steel strips. Such units are designed as shallow pickling installations, replacing previously known deep-pickling installations and comprise several shallow consecutively arranged pickling basins and use 18% preheated hydrochloric acid. The steel strips are pulled or pushed through the basins, wherein the layer of scale dissolves in the acid. The acid itself becomes less concentrated in the course of the pickling process so that a continuous supply of fresh acid is required.
DE-OS 36 29 894 discloses improvement in a shallow pickling installation, of rapid strip heating and media replacement in the boundary phase between the strip shaped rolled material and the treatment fluid, by disposing a specific treatment channel inside of the treatment basin. This channel encloses the strip shaped rolled material and is closed towards the top by a cover or lid. The treatment fluid is injected through nozzle beams disposed at the inlet and the outlet. At the inlet, the injection nozzles are directed in the direction of travel of the strip and directed counter to the direction of the travel of the strip at the outlet area. In order to assist the nozzle beams at the inlet and the outlet, additional side injections of the treatment fluid or the pickling agent can be disposed there also below the hot rolled strip. However, in this pickling installation, an undesirable long time period is required for heating the strip and the treatment fluid is only now uniformly applied to the strip.
The acid is introduced into the last pickling basin. Herein, the surface of the strip is to be subjected to conclusive treatment prior to leaving the pickling installation. The pickling agent is pumped out of the last basin, in counter flow, into the upstream basins so that acid with progressively reduced concentration arrives or is directed into the basins lying further upstream.
For minimizing the pickling time and for an increased throughput through the pickling installation, an adequate content of acid is maintained in the contact area between the strip surface and the pickling solution. Depletion of acid occurs by the pickling process in the contact regions. This depletion must be remedied by continuous replacement or exchange, meaning by circulation in the pickling basin. Injection of the pickling solution through the sidewall of the pickling basin, perpendicularly to the strip travel has been shown to be particularly effective. In a shallow pickler, disclosed, for instance in DE-OS 40 31 234, nozzles are arranged below the liquid level in the treatment basin both above and below of the material to be treated on both longitudinal sides of the basin. The nozzles inject the treatment liquid, in opposite directions, at an angle against the material to be treated. The injected acid causes turbulence in the pickling bath and improves the equalization of the acid concentration in the contact regions. Depending upon the speed of the strip moving through the pickling installation, a targeted influence of the turbulence in the basin can be desirable.
EP-OS 04 82 725 discloses varying turbulence in the basin as a function of the speed of the strip.
It is an object of the present invention however to create a method and an installation for improving the operational mode of a pickling installation of the above type and, in particular, a flexible adaptation to the respective pickling task with the aim of a reduced pickling time.